Heretofore, the prior art has disclosed catalysts for the polymerization of olefins, said catalysts comprising those known in the art as Ziegler-Natta catalysts. These catalysts typically consist of titanium tetrachloride which has been activated with an aluminum alkyl and operate in the form of a sludge or slurry. For example, U.S. Pat. No. 2,945,845 discloses a titanium tetrachloride catalyst which is used in conjunction with an organic compound such as triethyl aluminum or U.S. Pat. Nos. 3,660,419 and 3,725,497 which also teach the use of titanium tetrachloride catalysts which have been activated with the organoaluminum compounds. Titanium tetrachloride is reduced to lower valent titanium chloride by the aluminum alkyl. Other titanium halide catalysts which have been disclosed in the prior art include those such as shown in U.S. Pat. No. 3,153,634 in which a titanium tetrahalide is impregnated on an alumina such as gamma-alumina and is thereafter subjected to reducing conditions such as by treatment with hydrogen at temperatures ranging from 250.degree. to 500.degree. C. Alternatively, the impregnated alumina could be reduced by contact with a solution or dispersion of a reducing agent such as the alkali or alkaline earth metals or metal hydrides, etc. However, the catalysts thus prepared are useful in polymerizing olefins to form solid polymers, and specifically, high molecular weight solid polymers in which the molecular weight will range from 300 to 100,000. In addition to describing the preparation of solid polymers, this patent also states that aromatic hydrocarbons such as benzene, toluene, xylene or ethers may be used as suitable diluents. However, this is in contradistinction to the heterogeneous oligomerization catalyst of the type hereinafter set forth in greater detail in which it has been found that it is not possible to utilize aromatic hydrocarbons as diluents inasmuch as the olefinic hydrocarbon, if present, would enter into the reaction in which parts or all of the olefin would act as an alkylating agent rather than as a monomer in a polymerization reaction.
Another patent, namely U.S. Pat. No. 2,965,686, discloses a catalyst which is prepared by activating alumina by evacuation at a temperature of 600.degree. C. for a period ranging from about 18 hours to about 21.5 hours. After activation of the alumina, the base was then treated with a mixture of argon, an inert gas, and titanium tetrachloride vapor at a temperature of about 600.degree. C. for an unspecified period of time. The resultant catalyst was then used in an alkylation reaction for the propylation of cumene to form diisopropylbenzene. This patent is silent as to the type of alumina which was used as the base for the catalyst. In the process of the present invention, as will hereinafter be set forth in greater detail, it is believed that the substrate or base which is utilized must possess surface hydroxyl groups and therefore it is necessary to use specific types of alumina such as gamma-alumina, eta-alumina, etc. Another reference which discloses polymerization catalysts in U.S. Pat. No. 3,506,633 which teaches polymerization catalysts having a chlorine:titanium ratio of 2.5 to 3.5. However, in contradistinction to the catalyst of the present invention, this catalyst is used to prepare solid polymers. Yet another patent in this field is U.S. Pat. No. 2,381,481 in which the preparation and use of a catalyst prepared by treating alumina gel with fluotitanic acid is disclosed. However, as is the case of the previously mentioned patents, this catalyst is used to polymerize olefins to heavier hydrocarbons, i.e. solid polymers, and is also used to alkylate paraffins with olefins, usually at temperatures ranging between 700.degree. and 900.degree. F. or higher.
It has been known that Lewis acids such as metal halides can catalyze the Friedel Crafts type reactions and since Lewis acids alone show no or only mild catalytic activity, Lewis acids normally require cocatalyst. Titanium tetrahalides are well-known Lewis acids (Reference: A. G. Evans, G. W. Meadows and M. Polyani; Nature (London), 158, 94 (1946)).
In addition to these patents, two other U.S. patents also teach a process for oligomerizing olefinic compounds. U.S. Pat. No. 4,108,920 utilizes, as a catalyst for the reaction, a compound which has been prepared by heating a metal oxide which possesses surface hydroxyl groups with hydrogen and nitrogen at an elevated temperature which may range from about 350.degree. C. to about 550.degree. C. and thereafter impregnating the heated metal oxide with a solution of titanium tetrafluoride. The impregnated oxide is then cold-rolled, followed by steam drying and further drying the component at an elevated temperature of from about 200.degree. to about 600.degree. C. in an inert atmosphere. Likewise, U.S. Pat. No. 4,110,410 also discloses a process for the oligomerization of olefins using a catalyst which has been prepared by heating a metal oxide again possessing surface hydroxyl groups at a temperature in the range of from about 400.degree. to about 600.degree. C., contacting the metal oxide with a titanium tetrachloride vapor in a series of steps at progressively higher temperatures whereby the titanium tetrachloride is composited on the metal oxide. The impregnated metal oxide is then heated in contact with hydrogen at elevated temperatures ranging from about 300.degree. to about 700.degree. C. to reduce the titanium to a valence state of less than +4 and thereafter using this catalyst to oligomerize olefins. U.S. Pat. Nos. 4,048,108 and 4,048,109 disclose methods for preparing the catalysts which are utilized in the aforementioned two U.S. patents. However, the process using these catalysts results in a product mix which is heavy in highly branched chain products. As will hereinafter be shown in greater detail, by utilizing the catalysts of the present invention, it is possible to obtain selective oligomers of olefinic hydrocarbons, the oligomers being selectively less branched compounds.